Over-exposure to solar radiation is known to cause sunburn, skin damage and an increased risk of developing skin cancer, as well as photo-augmented skin aging. To safeguard against these risks, sun protection materials are applied to the skin, particularly for purposes of engaging in outdoor activities. In addition to their capacity to screen sunlight, the photoprotective compounds, which are typically organic molecules that protect against UV-A and/or UV-B rays, must also have good cosmetic properties, effective solubility in customary solvents, including both aqueous media and oils and fats, exhibit good resistance to water and perspiration (persistence) and satisfactory photostability. However, a large number of soluble organic filters can cause compatibility problems with the usual ingredients of cosmetic compositions mainly because of interactions with other filters or active organic agents such as antioxidants or vitamins that may be present, the result being that the photostability effect is not entirely satisfactory. Solving this latter problem is the subject of numerous patents, revealing how this problem is recurrent.
In addition to protecting the skin against exposure to UV light, the cosmetics industry also engages in continuous efforts to develop new formulas to lighten the skin. The substances most commonly used as bleaching agents include hydroquinone and its derivatives, kojic acid and its derivatives, azelaic acid, arbutin and its derivatives, alone or in combination with other active agents. These bleaching agents are not without drawbacks. For example, they frequently need to be used for a long time and in large amounts in order to obtain a bleaching effect on the skin. No immediate effect is observed on applying compositions comprising them. In addition, when hydroquinone and its derivatives are used in an amount that is effective to produce a visible bleaching effect, hydroquinone is known for its cytotoxicity towards melanocytes. Moreover, kojic acid and its derivatives can have the drawback of being expensive and consequently of not being able to be practicably used in large amounts in products for commercial mass distribution.
Other approaches to skin lightening entail application of cosmetic compositions capable of unifying the complexion, and which may give an immediate white appearance. These compositions include powders dispersed in a binder, wherein the powders are generally white or coloured pigments depending on the desired effect and/or fillers of various shapes (lamellar or spherical) depending on the desired effect. The drawback of such compositions is that the fading-out of the skin defects is provided by the covering power of the compositions. Skin that has thus been made up loses its natural look owing to the lack of transparency of these compositions.
To overcome these drawbacks, the use of optical brighteners such as stilbene derivatives, coumarin derivatives, oxazole and benzoxazole derivatives and imidazole derivatives has been proposed. Optical brighteners are bleaching agents that have fluorescent properties, absorbing in the ultraviolet range (maximum absorption at a wavelength of less than 400 nm) and re-emitting the energy by fluorescence at a wavelength of between 380 nm and 830 nm. An energy emission of between 400 nm and 480 nm results in an emission in the blue region of the visible field, which contributes, when this emission takes place on the skin, towards visually bleaching it. The optical brighteners are generally dispersed directly in the cosmetic formulations under consideration, which may, in certain cases, affect their physicochemical stability, and will do so proportionately more the greater their concentration in the compositions. Moreover, the introduction of these starting materials into compositions for topical use is limited by toxicological constraints. Since the lightening effect is directly linked to the amount of optical brightener used, it decreases when the concentration of optical brightener used is minimized.
Thus, there remains a need for cosmetic products for application to the skin that provide adequate protection against the hazards of over-exposure to U.V. radiation and to provide a skin lightening effect but which are safer to use.